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Blue oak trees unlock the secrets of California's current

An ancient blue oak (Quercus douglasii) stands alone at the lower forest border on the Windwolves Preserve, a large non-profit conservation property in California's San Joaquin Valley. Variations in atmospheric pressure over the northeast Pacific orchestrate the winds that drive productivity in the coastal ocean (upwelling) and the onshore flows of precipitation that drive tree growth on land. Given their sensitivity to moisture, blue oak unlock a long and exceptionally detailed history of upwelling in the California Current ecosystem.
[Copyright: Daniel Griffin, University of Minnesota]

Wind off the coast of California drives cool, nutrient-rich waters from the depths of the Pacific Ocean up to replace warm surface water in a process called coastal upwelling. Now, a new study shows that this upwelling off California's coast has become more variable over the past 60 years than almost any other time during the last 600 years.

Bryan Black and colleagues used tree rings from the stumps of blue oak trees, some of them more than 400 years old, as well as data on California's fish and seabirds, to paint a picture of the last 576 California winters. The researchers explain that upwelling in the California Current system depends on winter weather—and they used their tree ring record to predict how upwelling would have changed over those past six centuries.

Their results suggest that only two other periods of time—an interval during the late 19th century and another during the mid-20th century—could have hosted the kind of extreme variability that has been observed in the California Current's upwelling since about 1950. This recent trend has been bad news for fish, seabirds and mammals in the area, according to the researchers. But the long-term effects of this highly variable upwelling—whatever they may be—remains to be seen, they say.